Revert "Remove Gemfile"
Revert "Remove Gemfile"

This reverts commit 38f32913dc1766a169c346c15e1ce53dd13fe241.
Older rubies are not bundled with a recent enough minitest version, so
the tests fails. And the HEAD ruby versions also are not bundled with
minitest. So we need the Gemfile for travis to install an up to date
minitest.

DamienRobert/tiny_singleton